This invention relates in general to digital video signal processing, and more particularly to a method and apparatus for reducing the appearance of moirxc3xa9 in digitally resized images.
Cathode Ray Tube (CRT) monitors, particularly those used as computer displays, are usually capable of displaying images at various resolutions, for example 640xc3x97480, 800xc3x97600, 1024xc3x97768, etc. It is a relatively simple matter to create a CRT monitor that is capable of receiving a video signal at any of the above resolutions and displaying it such that the image fills the screen, regardless of the input resolution. This is possible since a CRT is an inherently analog display device and so an image of any given resolution can be made to fill the screen by appropriately controlling deflection of the electron beam.
Recently, conventional CRT displays have begun to be replaced by digitally addressable display devices such as Liquid Crystal Displays (LCDs), Plasma Display Panels (PDPs) and other technologies. Flat panel monitors employing LCD panels have become popular as replacements for CRT based computer monitors. LCD monitors have rapidly gained popularity due their compact size, perfectly flat screen and superior image quality. In order for an LCD monitor to function equivalently to a CRT monitor, the LCD monitor must be able to display a variety of different input formats and resolutions. Since the LCD panel is itself a fixed resolution device, it is necessary for an image of lower resolution to be digitally resized in order to fill the display. The process of scaling an image can lead to the production of moirxc3xa9 as a result of interference between the input and output sample rates. The moirxc3xa9 effect typically manifests itself as regions of varying intensity superimposed on the underlying image so as to detract from its overall appearance.
Scaling an image from one resolution to another, as required in an LCD monitor, involves a process known as interpolation, whereby new pixels are generated in accordance with their spatial position relative to the input pixels. In general, new pixels are generated by blending between the values of neighboring input pixels, where the weightings are dependent on the position of the new pixel relative to the input pixels. It is a desirable property of an image interpolator that the total energy at the output resulting from an impulse having been applied to the input, remain relatively constant irrespective of the position of the impulse. For example, if the input is a single white pixel on a black background, then the sum of all the non-black pixels at the output that result from the input, should be more or less constant regardless of the position of the white pixel in the input. This property is desirable since it implies that a feature in the input image will appear in the output image with an intensity that does not vary based on its position. It is the lack of preservation of is this property that is often responsible for the appearance of moirxc3xa9 in LCD monitors. Indeed, many high quality interpolation schemes, particularly those based on the well known (sin x)/x function, attempt to preserve this property. However, since the light output from the physical display device (LCD or other fixed resolution device) is typically not directly proportional to the level of signal that is input to the display, this property is not necessarily preserved at the eye of the observer, resulting in the appearance of moirxc3xa9. Consequently it is an object of the present invention, to provide a method and apparatus for reducing moirxc3xa9 in a digitally resized image.
The following patents are relevant as prior art relative to the present invention:
U.S. Patent Documents
According to the present invention, a method and apparatus are provided for reducing the appearance of moirxc3xa9 in a digitally resized image. According to the invention, each input pixel value is mapped to a new value by means of a first correction function prior to being resized. At the output of the resizing engine, each pixel in the resized image is mapped to a further new value by means of a second correction function prior to being displayed. The second correction function is chosen such that the intensity of light output from the display device is substantially proportional to the value of the signal from the resizing engine, taking into account the combined transfer functions of the second correction means and the physical display device (i.e. the second correction function is substantially the inverse of the display transfer characteristic). The first correction function is chosen to be substantially the inverse of the second function, or in this case, equal to the display transfer characteristic itself. As will be shown later, this has the effect of reducing moirxc3xa9 that would otherwise appear in the presence of high spatial frequencies, while leaving the overall DC transfer characteristic unaffected.
Alternatively, the degree to which moirxc3xa9 is reduced may be altered by replacing the second correction function as described above with one that closer approximates a straight line (the generation of such a function is described below). Regardless, the first correction function is still chosen to be substantially the inverse of the second correction function as described above, but would no longer be equal to the display transfer characteristic itself. This alternative allows the degree of moirxc3xa9 reduction to be controlled while still preserving the overall DC transfer characteristic.
Yet another variation allows the overall DC transfer characteristic to be intentionally modified so as to achieve a desired response. Such modification may be desirable when an input video signal has been preconditioned for one type of display device (for example, a CRT device) and is displayed on a device with a different light transfer characteristic (for example, an LCD device). In this embodiment, the net modification to the overall DC transfer characteristic is made by altering the first correction function, so as to achieve independence between the level of moirxc3xa9 reduction and the amount by which the overall DC transfer characteristic is modified.